Power factor is enhanced in high power factor power converters by constraining the input current waveform to conform to or replicate the waveform of the input voltage. This requires that the wave shape of the input current be accurately measured. The simplest technique is to use a current sensing resistor connected into the path of the input current. However, this provides a noisy measurement of current and does not readily provide metallic isolation from the sensed current. An alternative technique uses current transformers to sense the input current waveform and provide isolation. This technique requires artificial DC restoration of the current waveform to be added due to the DC blocking characteristic of the transformer in addition to requiring significant circuit board space.
The current transformer is also unidirectional in nature due to the need for DC restoration; hence, two oppositely poled transformers must be used to obtain the total bi-directional waveform. Further difficulties include the need for core reset of the transformer core and added multiple transformers required when bi-directional power switches are used in the circuitry.